Every presidential nominee going back to 2000 has revealed the names of influential supporters known as "bundlers" because of the way they persuade others to give money to a candidate. Every nominee, that is, until Mitt Romney.

The most anyone can give directly to any presidential campaign is $5,000, and everyone who gives that much is listed in the Romney campaign's monthly disclosures.

When it comes to the bundlers, though, the campaign chooses to keep those names secret.

Voluntary Disclosure

On one day last November, Mitt and Ann Romney had three different fundraising events in Florida. They ended up in Sarasota with 150 or so donors at the waterfront home of Bill and Christine Isaac.

"I would tell you that everyone was extremely impressed. I was," says Bill Isaac, who was chairman of the FDIC during the Reagan administration.

He says this year's race is his first foray into presidential politics. And he had never met Romney until that afternoon. The Romneys arrived, got some downtime at the house and then started mingling with the donors.

"It was an opportunity to see Mitt relaxed and really being himself," says Isaac.

Tickets cost $2,500 per person. The evening grossed about $375,000.

The campaign disclosed all of the donors to the Federal Election Commission, as required by law. But Bill and Christine Isaac, who brought those contributions into the campaign — they weren't identified anywhere.

The Romney campaign says it complies with the law, period. And there's no law requiring disclosure of bundlers. It has just been the voluntary standard since presidential candidate George W. Bush started doing it in 1999.

In 2008, Mitt Romney was among those adhering to the voluntary standard. But not this time.

Donors With A Past?

"The one danger is if Romney has a questionable bundler, you know, somebody with a troubled legal history," says Bill Allison, editorial director of the Sunlight Foundation, which advocates for transparency in government and politics.

Bundlers who carry political baggage have shown up along the campaign trail before — like Norman Hsu, a Ponzi scheme artist and onetime bundler for Hillary Clinton's 2008 campaign, or Jack Abramoff, the lobbyist who pleaded guilty to corruption charges after bundling for Bush in 2004.

"If that kind of story comes up, the question becomes about Romney is, 'Who are his other bundlers, and what is he hiding?' " says Allison.

And here's another reason to disclose: The public recognition can actually make the bundlers more enthusiastic and competitive.

The 2000 Bush campaign bragged about its "Pioneers" who each brought in $100,000 or more. And in 2008, candidate Barack Obama's organizationpromoted online bundling among small donors.

The Obama campaign this year has more than 500 "volunteer fundraisers," as it calls them. Some bring in $500,000 or more. The campaign identifies them on its website.

But on the other side, there is rising interest in defending a bundler's right to stay anonymous.

"The cost of disclosure has been always, and it's inevitable, is that the publicity is sometimes unwanted," says John Samples of the libertarian Cato Institute.

He says the argument against disclosure may be stronger in today's vitriolic political climate. Still, he says he's surprised that Romney is withholding the names.

"It seems like leading with your jaw in the sense that the president will have an easy time of fitting this in with the theme of the rich and so forth," says Samples.

Bill Isaac says he hadn't heard about Romney's nondisclosure policy.

"I make it very clear that I'm a very strong supporter of Mitt Romney and I'm doing everything I can possibly do to help him be our next president," says Isaac.

Now, $5,000 is the most that anyone can give directly to Mitt Romney's presidential campaign. And everyone who gives that much is listed in the Romney campaign's monthly disclosures. But when it comes to bundlers - those are supporters of Romney who solicit and bundle contributions from others - the campaign chooses to keep those names secret.

NPR's Peter Overby has more.

PETER OVERBY, BYLINE: On one day last November, Mitt and Ann Romney had three different fundraising events in Florida. They ended up in Sarasota with 150 or so donors at the waterfront home of Bill and Christine Isaac.

MITT ROMNEY: Bill, thank you so very much. Christine, I don't know, you much have moved furniture for days.

(SOUNDBITE OF LAUGHTER)

ROMNEY: I don't know where it went, but you have fit us all in here tonight and I thank you.

BILL ISAAC: It's a big ocean.

ROMNEY: It's a big ocean, yeah.

(SOUNDBITE OF LAUGHTER)

ISAAC: I would tell you that everybody was extremely impressed. I was.

OVERBY: Bill Isaac is in finance. He chaired the FDIC during the Reagan administration. He says this race is his first foray into presidential politics, and he'd never met the candidate till that afternoon. The Romneys arrived, got some downtime at the house, and then started mingling with the donors.

ISAAC: It was an opportunity to see Mitt relaxed and really being himself.

OVERBY: Tickets were $2,500 per person. The evening grossed about $375,000.

And here's how disclosure works: The campaign disclosed all of the donors to the Federal Election Commission. That's required by law. But Bill and Christine Isaac, who brought those contributions into the campaign, they aren't identified anywhere.

The Romney campaign says it complies with the law, period. And there's no law requiring disclosure of bundlers. It's just been the voluntary standard since presidential candidate George W. Bush started doing it in 1999. Among those adhering to the voluntary standard - Mitt Romney in 2008.

But not this time. Bill Allison is editorial director of the Sunlight Foundation, which advocates for transparency in government and politics.

BILL ALLISON: The one danger is that if Romney has a questionable bundler, you know, somebody with a troubled legal history...

OVERBY: Someone like Norman Hsu, a Ponzi scheme artist and one-time bundler for Hillary Clinton's 2008 campaign, or Jack Abramoff, the lobbyist who pleaded guilty to corruption charges after bundling for Bush in 2004.

ALLISON: If that kind of story comes up, the question becomes about Romney is, who are his other bundlers, and what is he hiding?

OVERBY: And here's another reason to disclose. Public recognition can actually make the bundlers more enthusiastic and competitive. The 2000 Bush campaign bragged about its pioneers, who each brought in $100,000 or more. And in 2008, candidate Barack Obama's organization promoted online bundling among small donors.

The Obama campaign this year has more than 500 volunteer fundraisers, as it calls them. Some bring in a half-million dollars or more. The campaign names them on its website. But on the right, there's rising interest in defending a bundler's right to stay anonymous. John Samples is with the libertarian Cato Institute.

JOHN SAMPLES: The cost of disclosure has been always, and it's inevitable, is that the publicity is sometimes unwanted.

OVERBY: Samples says the argument against disclosure may be stronger now in today's vitriolic political climate. Still, he says he's surprised that Romney is withholding the names.

SAMPLES: It seems like leading with your jaw in the sense that the president will have an easy time of fitting this in with the theme of the rich and so forth.